  In order to split the influence of the orbital and spin effects on the
inplane magnetoresistance of a quasi two-dimensional gas we derive its linear
response function and dielectric function for the case of anisotropic effective
mass. This result is used for the calculation of elastic transport relaxation
time of a quasi two dimensional system in a parallel magnetic field. The
relaxation time is proved to be isotropic in the low density limit for the case
of charged impurity scattering, allowing to separate the two contributions.
