Résumé
We have studied the effects of magnetic field on the fluctuations of the electron density of states in a disordered conductor, a contact in the resonant tunnelling structure GaAlAs/GaAs/GaAlAs. The local density of states (LDOS) in the contact is detected by an impurity which acts as a 'spectrometer'. In the regime of weak magnetic field, omega(c)tau less than or equal to 1, a linear increase of the fluctuation magnitude with B-2 is seen, due to the decrease of the volume where the LDOS is formed and a decrease in the statistical averaging. In the regime omega(c)tau greater than or equal to 1, large oscillations of the fluctuation magnitude have been observed. We suggest an explanation of this new effect, de Haas-van Alphen type oscillations in the fluctuation magnitude. The behaviour of impurity spectrometers in a magnetic field and images given by a complex spectrometer are also discussed.