  The one- and two-orbital double-exchange models for manganites are studied
using Monte Carlo computational techniques in the presence of a robust
electron-phonon coupling (but neglecting the antiferromagnetic exchange $J_{\rm
AF}$ between the localized spins). The focus in this effort is on the analysis
of charge transport. Our results for the one-orbital case confirm and extend
previous recent investigations that showed the presence of robust peaks in the
resistivity vs. temperature curves for this model. Quenched disorder
substantially enhances the magnitude of the effect, while magnetic fields
drastically reduce the resistivity. A simple picture for the origin of these
results is presented. It is also shown that even for the case of just one
electron, the resistance curves present metallic and insulating regions by
varying the temperature, as it occurs at finite electronic density. Moreover,
in the present study these investigations are extended to the more realistic
two-orbital model for manganites. The transport results for this model show
large peaks in the resistivity vs. temperature curves, located at approximately
the Curie temperature, and with associated large magnetoresistance factors.
Overall, the magnitude and shape of the effects discussed here closely resemble
experiments for materials such as $\rm La_{0.70} Ca_{0.30} Mn O_{3}$, and they
are in qualitative agreement with the current predominant theoretical view that
competition between a metal and an insulator, enhanced by quenched disorder, is
crucial to understand the colossal magnetoresistance (CMR) phenomenon.
