  The temperature dependence of 2D magnetoresistance in an applied in-plane
magnetic field is theoretically considered for electrons in Si MOSFETs within
the screening theory for long-range charged impurity scattering limited carrier
transport. In agreement with recent experimental observations we find an
essentially temperature independent magnetoresistivity for carrier densities
well into the 2D metallic regime due to the field-induced lifting of spin and,
perhaps, valley degeneracies. In particular the metallic temperature dependence
of the ballistic magnetoresistance is strongly suppressed around the
zero-temperature critical magnetic field ($B_s$) for full spin-polarization,
with the metallic temperature dependence strongest at B=0, weakest around $B
\sim B_s$, and intermediate at $B \gg B_s$.
