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Therefore, the $1 / T^2$ scaling of conductivity comes from combining two processes. The first process is one that is biased towards keeping the Fermi sphere intact, resulting in a decay time of quasiparticles with an energy $\epsilon$ above the Fermi energy proportional to $1 / \epsilon^2$. The second process deals with the loss of energy of the Fermi sphere itself, which happens via Umklapp scattering.  \subsection{Quasiparticle--impurity scattering}   Gives constant resistivity (dominates).     \subsection{Quasiparticle--phonon scattering}   Gives $T$ at high temperature (essentially due to the uncertainty principle) and $T^5$ at low temperature \cite{nozieres1999} (this is the only dependence I can't give an intuitive interpretation).