  A ferromagnetic phase, characterized by electron carriers and a high
temperature colossal magnetoresistance (HTCMR) dependent on the magnetic
moment, and a semiconducting phase, characterized by hole carriers and a low
temperature CMR (LTCMR), are observed in La$_{1-x}$Ca$_{x}$MnO$_3$ thin films
by the van der Pauw method. The LTCMR is much more sensitive to the magnetic
field than the HTCMR. In the ferromagnetic phase for films with anisotropic
moments in two dimensions, a remnant resistivity of the order of 10$^{-8}
\~{\Omega}m$ is observed up to 100 K and increases exponentially with both a
temperature up to $T_c$ and a magnetic field above one Tesla (a positive
magnetoresistivity). We found that the ferromagnetic phase below $T_c$ is in a
polaronic state with a polaronic mobile conduction, and the carrier density
dips near $T_c$. For resistances measured by the four-probe method with line
electrodes, low temperature information of the HTCMR is not revealed. The van
der Pauw method is more effective for the resistance measurement of a magnetic
material than the four-probe method.
