We provide a possible theoretical explanation for the recently observed giant positive magnetoresistance in high mobility low density { \it quasi}-two dimensional electron and hole systems. Our explanation is based on the strong coupling of the parallel field to the { \it orbital} motion arising from the { \it finite} layer thickness and the large Fermi wavelength of the { \it quasi}-two dimensional system at low carrier densities.
Categories: cond-mat.str-el cond-mat.dis-nn