In this work we report experimental studies of the critical magnetic field for indium films of different thicknesses alloyed with 2 at.% and 3 at.% of tin, thus furnishing, for the first time, experimental data of Hc～d for the case of finite electronic mean free path. From these data, it is concluded that the critical magnetic field of thin films can only be discussed in the light of the non-local, nonlinear theories, and that in the thin film limit, Hc∝d-3/2 for both pure and alloyed films. The local theories of london and of Гинзбург-Ландау, and the non-local, linear theory of Ittner, are inapplicable. The erroneous conclusion of Hc∝d-1 made by certain outhors, is due to their incorrect analyses of their experimental data.