Capacitance-voltage (C-V) analysis of high quality MBE grown quantum well samples shows that carrier distributions are averaged over the scale of the Debye length. This averaging process results in strongly temperature dependent C-V-deduced doping distributions that can be very different from the actual ones. The doping distribution of the structure is obtained by fitting numerically simulated curves to the measured C-V curves and doping profiles, respectively. Although the calculations do not take quantum size effects into account they show good agreement with the measured data.