  We consider quantum lifetime derived from low-field Shubnikov-de Haas
oscillations as a metric of quality of the two-dimensional electron gas in GaAs
quantum wells that expresses large excitation gaps in the fractional quantum
Hall states of the N=1 Landau level. Analysis indicates two salient features:
1) small density inhomogeneities dramatically impact the amplitude of
Shubnikov-de Haas oscillations such that the canonical method (cf. Coleridge,
Phys. Rev. B \textbf{44}, 3793) for determination of quantum lifetime
substantially underestimates $\tau_q$ unless density inhomogeneity is
explicitly considered; 2) $\tau_q$ does not correlate well with quality as
measured by $\Delta_{5/2}$, the excitation gap of the fractional quantum Hall
state at 5/2 filling.
