Two-dimensional metal in a parallel magnetic field

We have investigated the effect of an in-plane parallel magnetic field (B-parallel to) on two high mobility metalliclike dilute two-dimensional hole gas systems in GaAs quantum wells. The experiments reveal that, while suppressing the magnitude of the low temperature resistance drop, B-parallel to does not affect E-a, the characteristic energy scale of the metallic resistance drop. The field B-c at which the metalliclike resistance drop vanishes is dependent on both the width of the quantum well and the orientation of B-parallel to. It is unexpected that E-a is unaffected by B-parallel to up to B-c despite the fact that the Zeeman energy at B-c is roughly equal to E-a.