  In developing the phonon quasiparticle picture, Peierls discovered that, in a
perfect crystal, without Umklapp (U) events, a current-carrying distribution
can never relax to a zero-current distribution. Callaway introduced a
simplified approximate model version of the Peierls-Boltzmann equation,
retaining its the ability to deal separately with Normal (N) and U events. This
paper clarifies and improves the Callaway model, and shows that Callaway
underestimated the suppression of N-processes in relaxing thermal current. The
new result should improve computations of thermal conductivity from
relaxation-time studies.
