= Normal phonon collisions and heat-current relaxation
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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