The local density of states of heavily doped GaAs is explored at high magnetic fields, where only a single or few Landau bands are occupied. Our experiment is based on resonant tunneling through impurity states and images the local density of states both below and above the Fermi level. Fan-type mesoscopic fluctuations are observed in the energy-magnetic-field plane, which we attribute to the interplay of Landau quantization and quantum interference of scattered electron waves in the disordered conductor. Our conclusion is supported by the suppression of the fluctuations high above the Fermi level, where dephasing due to inelastic processes is as fast as elastic scattering.