Within a one plane wave spherical Fermi surface model it is shown that the change in scattering time anisotropy resulting from the addition of small amounts of impurities to pure Zn leads to measurable changes in the temperature dependence of the resistivity ratio rho /// rho perpendicular to ; rho //( rho perpendicular to ) is the electrical resistivity with the electric field parallel (perpendicular) to the c axis. The corresponding deviations from Matthiessen's rule for polycrystalline samples are also calculated.