= Nernst-Einstein relation
{c}
{title2=$\sigma=nq^2D/(k_BT)$}
For dilute, independently migrating carriers of number density $n$, charge $q$ and <diffusion coefficient> $D$, balancing drift with <diffusion> gives mobility $D/(k_BT)$ per applied force. Multiplying drift velocity by charge density yields $\sigma=nq^2D/(k_BT)$. State which species defines $n$ and $D$: a vacancy diffusivity must be combined with vacancy density. Correlated motion requires a correction rather than automatic use of a tracer diffusivity.
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