  We have explored phase coherent transport of holes in two p-type GaAs quantum
rings with orbital radii 420 nm and 160 nm fabricated with AFM oxidation
lithography. Highly visible Aharonov-Bohm (AB) oscillations are measured in
both rings, with an amplitude of the oscillations exceeding 10% of the total
resistance in the case of the ring with a radius of 160 nm. Beside the h/e
oscillations, we resolve the contributions from higher harmonics of the AB
oscillations. The observation of a local resistance minimum at B=0 T in both
rings is a signature of the destructive interference of the holes' spins. We
show that this minimum is related to the minimum in the h/2e oscillations.
