Using resonant-tunneling spectroscopy, we have studied the energy levels in wide quantum wells (of 60 and 120 nm well width) of double-barrier resonant-tunneling devices in high magnetic fields with a small tilt angle-alpha with respect to the growth axis. In the limit of high magnetic fields, the resonance spacing between successive levels becomes independent of the magnetic field and is cos2-alpha times the spacing at zero field. In this limit, these results can be understood quasiclassically by considering the motion of the particles to follow the magnetic-field lines inside of a quantum well with an effective thickness w/cos-alpha.