where ∆ f ave is the average intraband spacing and f max is the maximum frequency in the dispersion. This condition is close to being met in both π SnS (∆ f ave = 4.4 × 10 -2 THz, f max /(3n a ) = 4 × 10 -2 THz) and SnSe (2.9 × 10 -2 THz, 2.5 × 10 -2 THz). Fig. (a) shows the contributions of the RTA κ p and intraband tunnelling κ w , obtained by solving the Wigner transport equation, to the κ latt of π SnSe. The κ p and κ w become equal around 200 K and the κ w accounts for 78% of the κ latt at 700 K. In contrast, the κ w of Pnma SnSe is almost negligible at 300 K and increases the κ latt by 18% at 700 K (Fig. ). π and Pnma SnS show similar, contrasting, behaviour (Figs S4/S5 † ). The very low κ p confirms the strategy of targeting large, complex unit cells can reduce the κ p below the amorphous limit. A consequence of the shallow temperature dependence of the κ w is that π SnS and SnSe have 70-75% lower thermal conductivity than the Pnma phases at 300 K, and κ latt comparable to the layering b direction, which is the "hard" axis for thermal transport, at high T (Fig. ).