It has been suggested that the isotopic composition, the ratio of 12C to 13C, has a significant impact on the thermal conductivity. For example, isotopically pure 12C graphene has higher thermal conductivity than either a 50:50 isotope ratio or the naturally occurring 99:1 ratio."Lay summary": </ref> It can be shown by using the Wiedemann–Franz law, that the thermal conduction is phonon-dominated. However, for a gated graphene strip, an applied gate bias causing a Fermi energy shift much larger than "k"B"T" can cause the electronic contribution to increase and dominate over the phonon contribution at low temperatures. The ballistic thermal conductance of graphene is isotropic.