Conductance oscillations in mesoscopic rings: Microscopic versus global phase

The phase of Aharonov-Bohm oscillations in mesoscopic diffusive metal rings in the presence of a magnetic field, can be modulated by the application of a dc-bias current Idc. We address the question of how a variation of Idc and hence of the microscopic phases of the electronic wave functions results in the observable ``global'' phase of the conductance oscillations. Whereas the first one can be varied continuously, the latter has to be quantized for a ring in a two-wire configuration by virtue of the Onsager symmetry relations. We observe a correlation between a phase flip by ±π and the amplitude of the oscillations.