Fig. S7.

$\alpha =(D\left(T\right)/G\left(T\right))[1-\exp (-\mathrm{\Delta }{G}_{lx}\left(T\right)/k_{b}T)]$ vs. temperature using previously reported results for D(T) from Gillen et al. (57) and Price et al. (30), and G(T) determined from our pulsed-heating measurements (Fig. 2). The data of Gillen et al. and Price et al. extend down to 242.5 and 237.8 K, respectively.