  We investigate theoretically the collective charge density wave motion
through an ensemble of small disordered Aharonov-Bohm rings. It is shown that
the magnetic flux modulates the threshold field and the magnetoresistance with
a half flux quantum periodicity $\Phi_{0}/2=h/2e$, resulting from ensemble
averaging over random scattering phases of multiple rings. The magnitude of the
magnetoresistance oscillations decreases rapidly with increasing bias. This is
consistent with recent experiments on $NbSe_3$ in presence of columnar defects
[Phys. Rev. Lett. 78, 919 (1997)].
