  Mesoscopic solid state Aharonov-Bohm interferometers have been used to
measure the "intrinsic" phase, $\alpha_{QD}$, of the resonant quantum
transmission amplitude through a quantum dot (QD). For a two-terminal "closed"
interferometer, which conserves the electron current, Onsager's relations
require that the measured phase shift $\beta$ only "jumps" between 0 and $\pi$.
Additional terminals open the interferometer but then $\beta$ depends on the
details of the opening. Using a theoretical model, we present quantitative
criteria (which can be tested experimentally) for $\beta$ to be equal to the
desired $\alpha_{QD}$: the "lossy" channels near the QD should have both a
small transmission and a small reflection.
