  Magnetophonon resonance in quantum wells in a tilted magnetic field B is
investigated. Measurements of the Hall coefficient and correspondingly of the
carrier concentration as a functions of magnetic field and temperature are
simultaneously performed. It is shown that the experimental data can be
interpreted in terms of a great sensitivity of the effect to the variation of
the two dimensional carrier concentration n in a certain concentration
interval. In other words, the observed angular dependence of the MPR amplitudes
is a manifestation of dependence of n on the magnitude of the magnetic field B.
