  Oscillations of the real component of AC conductivity $\sigma_1$ in a
magnetic field were measured in the n-AlGaAs/GaAs structure with a wide (75 nm)
quantum well by contactless acoustic methods at $T$=(20-500)~mK. In a wide
quantum well, the electronic band structure is associated with the two-subband
electron spectrum, namely the symmetric (S) and antisymmetric (AS) subbands
formed due to electrostatic repulsion of electrons. A change of the
oscillations amplitude in tilted magnetic field observed in the experiments
occurs due to crossings of Landau levels of different subbands (S and AS) at
the Fermi level. The theory developed in this work shows that these crossings
are caused by the difference in the cyclotron energies in the S and AS subbands
induced by the in-plane magnetic field.
