  The phase of the transmission amplitude through a mesoscopic system contains
information about the system's quantum mechanical state and excitations
thereof. In the absence of an external magnetic field, abrupt phase lapses
occur between transmission resonances of quantum dots and can be related to the
signs of tunneling matrix elements. They are smeared at finite temperatures. By
contrast, we show here that in the presence of a strong magnetic field, phase
lapses represent a genuine interaction effect and may occur also on resonance.
For some realistic parameter range these phase lapses are robust against finite
temperature broadening.
