  A negative magnetoresistance under the in-plane magnetic field, reaching
maximum 30-40% of its zero-field value in fields higher than ~12 T, has been
found in wide Ge_{1-x}Si_x/Ge/p-Ge_{1-x}Si_x quantum wells (QW) containing the
quasi-two-dimensional hole gas. In the QWs of intermediate widths and hole
densities, this negative magnetoresistance may be explained as being caused by
suppression of the intersubband scattering due to the upper subband
depopulation. For the widest QWs with the highest hole densities, in which the
hole gas is divided into two sublayers, similar negative magnetoresistance was
observed and tentatively interpreted as also been due to suppression of the
intersubband scattering, but subbands are the lowest symmetric and
antisymmetric states of the double quantum well structure. These subbands shift
under the in-plane magnetic field not vertically in energy, but horizontally
along the wave vector.
