At high temperatures (T > Θ), the mean free path and therefore λ L has a temperature dependence T, to which one arrives from formula e Θ Θ / b T by making the following approximation e x ∝ ∝ x , ( x ) < 1 and writing x = Θ Θ / b T . This dependency is known as Eucken's law and originates from the temperature dependency of the probability for the U-process to occur.