  The longitudinal magnetoresistance (MR) is assumed to be hardly realized as
the Lorentz force does not work on electrons when the magnetic field is
parallel to the current. However, in some cases, longitudinal MR becomes large,
which exceeds the transverse MR. To solve this problem, we have investigated
the longitudinal MR considering multivalley contributions based on the
classical MR theory. We have showed that the large longitudinal MR is caused by
off-diagonal components of a mobility tensor. Our theoretical results agree
with the experiments of large longitudinal MR in IV-VI semiconductors,
especially in PbTe, for a wide range of temperatures, except for linear MR at
low temperatures.
