Normal phonon collisions conserve the crystal momentum of the distribution, whereas Umklapp scattering transfers momentum by a reciprocal vector. A normal collision can redistribute heat-carrying modes without relaxing a drifting distribution in the same manner as a momentum-relaxing event. Boundaries, impurities and Umklapp processes can then set the resistance. Counting all collision rates as interchangeable inverse transport times can miss collective phonon flow and nontrivial current weighting.
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