  Given the large anisotropy of transport processes in magnetized plasmas, the
magnetic field structure can strongly impact heat diffusion: magnetic surfaces
and cantori form barriers to transport while chaotic layers and island
structures can degrade confinement. When a small but non-zero amount of
perpendicular diffusion is included, the structure of the magnetic field
becomes less important, allowing pressure gradients to be supported across
chaotic regions and island chains. We introduce a metric for the effective
volume over which the local parallel diffusion dominates based on the solution
to the anisotropic heat diffusion equation. To validate this metric, we
consider model fields with a single island chain and a strongly chaotic layer
for which analytic predictions of the relative parallel and perpendicular
transport can be made. We also analyze critically chaotic fields produced from
different sets of perturbations, highlighting the impact of the mode number
spectrum on the heat transport. Our results indicate that this metric coincides
with the effective volume of non-integrability in the limit $\kappa_{\perp}
\rightarrow 0$. We propose that this metric be used to assess the impact of
non-integrability on the heat transport in stellarator equilibria.
