  We present evidence of strong Shubnikov-de-Haas magnetoresistance
oscillations in a polarization-doped degenerate three-dimensional electron slab
in an Al$_{x}$Ga$_{1-x}$N semiconductor system. The degenerate free carriers
are generated by a novel technique by grading a polar alloy semiconductor with
spatially changing polarization. Analysis of the magnetotransport data enables
us to extract an effective mass of $m^{\star}=0.19 m_{0}$ and a quantum
scattering time of $\tau_{q}= 0.3 ps$. Analysis of scattering processes helps
us extract an alloy scattering parameter for the Al$_{x}$Ga$_{1-x}$N material
system to be $V_{0}=1.8eV$.
