= Matthiessen's rule
{c}
<Matthiessen's rule> approximates the total <electrical resistivity> of a <metal> by the sum of separate scattering contributions. For a common parabolic <energy band>, approximately independent scattering mechanisms give $1/\tau=\sum_j1/\tau_j$ for the respective <momentum relaxation times>, and hence $\rho_{\rm el}=m/(ne^2\tau)=\sum_jm/(ne^2\tau_j)$. Correlated scattering, strongly angle-dependent processes or a complicated changing <energy band> structure can invalidate the simple addition.
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