  The phase of Aharonov-Bohm oscillations in mesoscopic metal rings in the
presence of a magnetic field can be modulated by application of a DC-bias
current I_DC. We address the question of how a variation of I_DC and hence of
the microscopic phases of the electronic wave functions results in the
macroscopic phase of the conductance oscillations. Whereas the first one can be
varied continuously the latter has to be quantized for a ring in two-wire
configuration by virtue of the Onsager symmetry relations. We observe a
correlation between a phase flip by +/- pi and the amplitude of the
oscillations.
