Two-dimensional metal in a parallel magnetic field

We have investigated the effect of an in plane parallel magnetic field (B_||) on two high mobility metallic-like dilute two-dimensional hole gas (2DHG) systems in GaAs quantum wells. The experiments reveal that, while suppressing the magnitude of the low temperature resistance drop, B_|| does not affect E_a, the characteristic energy scale of the metallic resistance drop. The field B_c at which the metallic-like resistance drop vanishes is dependent on both the width of quantum well and the orientation of B_||. It is unexpected that E_a is unaffected by B_|| up to Bc depite that the Zeeman energy at B_c is roughly equal to E_a.