  We report the observation of ballistic commensurability oscillations and
positive magnetoresistance in a high-mobility, (001) GaAs two-dimensional hole
system with a unidirectional, surface-strain-induced, periodic potential
modulation. The positions of the resistivity minima agree well with the
electrostatic commensurability condition. From an analysis of the amplitude of
the oscillations we deduce a ballistic scattering time and an effective
magnitude for the induced periodic potential seen by the two-dimensional holes.
