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We consider a momentum dependent relaxation time for the Boltzmann equationin the relaxation time approximation. We employ a power law parametrization forthe momentum dependence of the relaxation time, and calculate the shear andbulk viscosity, as well as, the charge and heat conductivity. We show, that forthe two popular parametrizations, referred to as the linear and quadraticansatz, one can obtain transport coefficients which corresponds to the weak andstrong coupling regimes, respectively. We also show that, for a system ofmassless particles with vanishing chemical potential, the off-equilibriumcorrections to the phase-space distribution function calculated with thequadratic ansatz are identical with those of the Grad's 14-moment method.

Weak and strong coupling limits of the Boltzmann equation in the relaxation-time approximation