  Ta-doped SnO$_2$ films with high conductivity and high optical transparency
have been successfully fabricated using rf-sputtering method and their
electrical transport properties have been investigated. All films reveal
degenerate semiconductor (metal) characteristics in electrical transport
properties. For the thick films ($t\sim 1\,\mu \rm{m}$ with $t$ being the
thickness) deposited in pure argon, the electron-phonon scattering alone cannot
explain the temperature dependent behaviors of resistivity, the interference
effect between electron-phonon and electron-impurity scattering should be
considered. For the $t\lesssim 36$ nm films, both conductivity and the Hall
coefficient show linear relation with the logarithm of temperature ($\ln T$)
from $\sim$100 K down to liquid helium temperature. The $\ln T$ behaviors of
conductivity and Hall coefficient cannot be explained by the Altshuler-Aronov
type electron-electron interaction effect, but can be quantitatively
interpreted by the electron-electron interaction effects in the presence of
granularity. Our results not only provide strong supports for the theoretical
results on the electron-phonon-impurity interference effect, but also confirm
the validity of the theoretical predictions of charge transport in granular
metals in strong coupling regime.
