Solution
= Solution
Both coexisting phases transport the conserved order parameter. The calculation above included only the harmonic field and normal current in the upper half-space. The lower half-space contributes an equal flux into the interface for equal mobilities and symmetric phases. The total interface speed is therefore twice the one-sided result, so the correct coefficient is
$$
\boxed{\nu_{\rm full}=\frac{M\sigma}{2\phi_B^2}}.
$$