Therefore, these phonons have to possess energy of ∼ ∼ k Θ Θ / 2 , which is a significant fraction of Debye energy that is needed to generate new phonons. The probability for this is proportional to e − − Θ Θ / b T , with b = 2 . Temperature dependence of the mean free path has an exponential form e Θ Θ / b T . The presence of the reciprocal lattice wave vector implies a net phonon backscattering and a resistance to phonon and thermal transport resulting finite λ L, as it means that momentum is not conserved. Only momentum non-conserving processes can cause thermal resistance.