  We address the quantum dot phase measurement problem in an open Aharonov-Bohm
interferometer, assuming multiple transport channels. In such a case, the
quantum dot is characterized by more than one intrinsic phase for the electrons
transmission. It is shown that the phase which would be extracted by the usual
experimental method (i.e. by monitoring the shift of the Aharonov-Bohm
oscillations, as in Schuster {\it et al.}, Nature {\bf 385}, 417 (1997)) does
not coincide with any of the dot intrinsic phases, but is a combination of
them. The formula of the measured phase is given. The particular case of a
quantum dot containing a $S=1/2$ spin is discussed and variations of the
measured phase with less than $\pi$ are found, as a consequence of the
multichannel transport.
