  In the 2016 experiment by Crossno et al. [Science 351, 1058 (2016)],
electronic contribution to the thermal conductivity of graphene was found to
violate the well-known Wiedemann-Franz (WF) law for metals. At liquid nitrogen
temperatures, the thermal to electrical conductivity ratio of charge-neutral
samples was more than 10 times higher than predicted by the WF law, what was
attributed to interactions between particles leading to collective behavior
described by hydrodynamics. Here we show, by adapting the handbook derivation
of the WF law to the case of massless Dirac fermions, that significantly
enhanced thermal conductivity should appear also in few- or even sub-kelvin
temperatures, where the role of interactions can be neglected. The comparison
with numerical results obtained within the Landauer-B\"uttiker formalism for
rectangular and disk-shaped (Corbino) devices in ballistic graphene is also
provided.
