  Presently, cerium-doped LaMnO$_3$ is vividly discussed as an electron-doped
counterpart prototype to the well-established hole-doped mixed-valence
manganites. Here, La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films of different
thicknesses, degrees of CeO$_2$ phase segregation, and oxygen deficiency, grown
on SrTiO$_3$ single crystal substrates, are compared with respect to their
resistance-vs.-temperature (R vs. T) behavior from 300~K down to 90~K. While
the variation of the film thickness (and thus the degree of epitaxial strain)
in the range between 10~nm and 100~nm has only a weak impact on the electrical
transport, the degree of oxygen deficiency as well as the existence of CeO$_2$
clusters can completely change the type of hopping mechanism. This is shown by
fitting the respective \textit{R-T} curves with three different transport
models (adiabatic polaron hopping, Mott variable-range hopping,
Efros-Shklovskii variable-range hopping), which are commonly used for the
mixed-valence manganites. Several characteristic transport parameters, such as
the hopping energies, the carrier localization lengths, as well as the Mn
valences are derived from the fitting procedures.
