[en] Empirical scaling laws for the electron heat diffusivity χ_e in the OH, L and H regimes are expressed in dimensionless form. It is found that particle orbit, collisionality and finite beta effects enter in all regimes. The different scalings are shown to result from changes in the finite β contribution, i.e. in magnetic turbulence induced transport. A unified scaling law χ_e∝a^-1/2 A_i^-1/2 n_e^α-1/2 T_e^α-1/2 (q/B_t)^2α with α=-1/2 (OH) and α=1/2 (L and H) is presented which also holds for the intermediate region between OH and L confinement. Purely electrostatic drift wave models including gyrokinetic effects and collisions yield B_t, A_i and T_e scalings incompatible with the empirical results. Inclusion of electromagnetic effects and pressure-driven terms, e.g. due to resistive ballooning modes, is shown to be necessary. (orig.)