  We analyze the magnetoconductance of two-dimensional electron and hole gases
(2DEGs) subject to a parallel magnetic field. It is shown that, for confining
potential wells which are symmetric with respect to spatial inversion, a
temperature-dependent weak localization signal exists even in the presence of a
magnetic field. Deviations from this symmetry lead to magnetoconductance
profiles that contain information on both, the geometry of the confining
potential and characteristics of the disorder.
