Effects of a parallel magnetic field on the novel metallic behavior in two dimensions

Magnetoconductance (MC) in a parallel magnetic field B has been measured in a two-dimensional electron system in Si, in the regime where the conductivity decreases as \sigma (n_s,T,B=0)=\sigma (n_s,T=0) + A(n_s)T^2 (n_s -- carrier density) to a non-zero value as temperature T->0. Very near the B=0 metal-insulator transition, there is a large initial drop in \sigma with increasing B, followed by a much weaker \sigma (B). At higher n_s, the initial drop of MC is less pronounced.