= Solution
\b[Yes, for all three ideal lattices under the stated assumptions.] Cubic symmetry and equal rates make $\boldsymbol\sigma=\sigma I$ with $\sigma>0$, so $\mathbf j=\sigma\boldsymbol{\mathcal E}$. Positive <ions> drift along the field. The current coefficient contains $Q^2$, so negative charge carriers would also produce a conventional current along the field, although their particle drift would reverse. Anisotropic barriers or a field strong enough to invalidate <linear response> would require a new calculation.
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