  We propose a controlled experiment to clarify the physical mechanism causing
phase lapses of the amplitude for electron transmission through nanoscale
devices. Such lapses are generically observed in valleys between adjacent
Coulomb--blockade peaks. The experiment involves two quantum dots embedded in
the same arm of an Aharonov--Bohm interferometer. It offers a decisive test of
"population switching", one of the leading contenders for an explanation of the
phenomenon.
