  We report the measurements and analysis of weak antilocalization (WAL) in
Pb1-xSnxSe topological quantum wells in a new regime where the elastic
scattering length is larger than the magnetic length. We achieve this regime
through the development of high-quality epitaxy and doping of topological
crystalline insulator (TCI) quantum wells. We obtain elastic scattering lengths
that exceeds 100nm and become comparable to the magnetic length. In this
transport regime, the Hikami-Larkin-Nagaoka model is no longer valid. We employ
the model of Wittmann and Schmid to extract the coherence time from the
magnetoresistance. We find that despite our improved transport characteristics,
the coherence time may be limited by scattering channels that are not strongly
carrier dependent, such as electron-phonon or defect scattering.
