It is shown that a thermal diffusion coefficient (λD Debye length; ωpi ion plasma frequency; ωpe electron plasma frequency; ωce electron cyclotron frequency) gives a reasonable explanation for the empirical scaling law of the energy confinement times of tokamaks, including the recent ISX-A experiment with its relatively low toroidal magnetic field. In the limit of ωpe > ωce, the confinement time becomes independent of the plasma density; this has also been observed in ISX-A. In the same limit, D⊥ essentially becomes the Bohm diffusion coefficient.