  Suppression of the critical temperature in homogeneously disordered
superconducting films is a consequence of the disorder-induced enhancement of
Coulomb repulsion. We demonstrate that for the majority of thin films studied
now this effect cannot be completely explained in the assumption of
two-dimensional diffusive nature of electrons motion. The main contribution to
the $T_c$ suppression arises from the correction to the electron-electron
interaction constant coming from small scales of the order of the Fermi
wavelength that leads to the critical temperature shift $\delta T_c/T_{c0} \sim
- 1/k_Fl$, where $k_F$ is the Fermi momentum and $l$ is the mean free path.
Thus almost for all superconducting films that follow the fermionic scenario of
$T_c$ suppression with decreasing the film thickness, this effect is caused by
the proximity to the three-dimensional Anderson localization threshold and is
controlled by the parameter $k_F l$ rather than the sheet resistance of the
film.
