  We have obtained numerically exact results for the spin-related geometric
quantum phases that arise in p-type semiconductor ring structures. The
interplay between gate-controllable (Rashba) spin splitting and
quantum-confinement-induced mixing between hole-spin states causes a much
higher sensitivity of magnetoconductance oscillations to external parameters
than previously expected. Our results imply a much-enhanced functionality of
hole-ring spin-interference devices and shed new light on recent experimental
findings.
