Moreover, the above derivation in the Landau gauge assumed an electron confined in the z -direction, which is a relevant experimental situation — found in two-dimensional electron gases, for instance. Still, this assumption is not essential for the results. If electrons are free to move along the z direction, the wave function acquires an additional multiplicative term exp(ik z z) ; the energy corresponding to this free motion, (ħ k z) /(2 m), is added to the E discussed. This term then fills in the separation in energy of the different Landau levels, blurring the effect of the quantization. Nevertheless, the motion in the x - y -plane, perpendicular to the magnetic field, is still quantized.