It is found that phase shifts in the conductance oscillations of a two-dimensional (2D) electron gas in a tilted magnetic field may result from the correction to the electron energy levels due to the parallel component of the magnetic field. Such phase shifts are observed in a GaAs/Ga0.7Al0.3As heterojunction. From the size of the shifts, one can calculate the standard deviation in the position of the electron, normal to the 2D plane. The effective width of the electronic wave function determined in this way is 70 AI. © 1985 The American Physical Society.