Shape of LAC curve:Empirical envidence

According to the traditional theory,the long-run average cost curve is also 'U' shaped like the short run average cost curve.But some economists have found from empirical study that the LAC curve is L shaped rather than being 'U' shaped .The LAC curves first fall rapidly in the beginning.But after a point it becomes fully flat or may slopes downward slowly.Such a L shaped LAC curve is shown in figure below.
According to most of the empirical studies there is increasing returns to scale in lower level of production.But when output increases,the economies of scale decline and the consist returns to scale operates in higher level of production.This implies that as the output increase,the LAC curve slopes downwards at the diminishing rate and finally becomes horizontal.The operation of decreasing returns to scale or output of average cost curve is rarely found.
P.J.D.Wiles has concluded on the basis of his that the average cost like the branch of left side of capital letter 'U' first rapidly and then slowly slopes downwards.The decline in cost with size is almost universal.But 'U' never turns upward.The rapid increase in cost with size is practically unknown and even little rise as rare.He remarks"Most of the cost function obey what we may call the L-shaped costs."Johnston has also derived same conclusion from his study.From the study of cost estimate of electricity production in Great Britain,he has found the average cost to decline in the beginning and then becoming leveled off.
The reasons for the LAC curve being L shaped are as follows :

1. Technological progress :In economics theory,technological is assumed to be constant.But technology changes in real life.Due to this,the average cost decline and does not rise.Suppose that the firm in the beginning produces quality at cost on working at curve.If we connect the minimum point of long run cost curve which is lower than average cost.
2. Learning by doing :The LAC curve completely slopes completely downwards due to learning by doing.Since the efficiently of firm increases due to continuing the work,it is able to reduce cost.As the output is increased,there is not only the increase in knowledge of many things,there is also an improvement in the management of plant.Due to this LAC curve is L shaped.
3. Management technique :According to the modern management theory,appropriate administrative structure is available to operate the plant of each size.There exists appropriate management technique in different levels of management.The management technique is available in large and small size.The cost of different management first fall up to certain plant size.The managerial cost slowly increase slowly up to very large level of output.