[en] The oscillatory magnetoresistance (Shubnikov-de Hass) effect has been used to investigate the two-dimensional electron gas in Al_xGa_1-xAs/GaAs single quantum wells and heterostructures grown by molecular beam epitaxy. The electron carrier density of the subband can be obtained clearly by digitizing the data linearly in terms of 1/B and fast Fourie transformation of the data by computer. Comparisons between these analyses and the period of oscillation of each subband obtained from the slope of a plot of reciprocal magnetic field vs the number of oscillation peaks for these samples will be discussed, as will determination of the mobility of the subband using the onset position of the oscillations and a plot ln(amplitude) vs I/B. (Author)