Two-Dimensional Metal in a Parallel Magnetic Field

We have investigated the effect of an in-plane parallel magnetic field $\left(B_{\Vert }\right)$ 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, $B_{\Vert }$ 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 $B_{\Vert }$. It is unexpected that $E_a$ is unaffected by $B_{\Vert }$ up to $B_c$ despite the fact that the Zeeman energy at $B_c$ is roughly equal to $E_a$.