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We performed numerical simulations of the $q$-state Potts model to compute the reduced conductivity exponent $t/ u$ for the critical Coniglio-Klein clusters in two dimensions, for values of $q$ in the range $[1;4]$. At criticality, at least for $q<4$, the conductivity scales as $C(L) sim L^{-frac{t}{ u}}$, where $t$ and $ u$ are, respectively, the conductivity and correlation length exponents. For q=1, 2, 3, and 4, we followed two independent procedures to estimate $t / u$. First, we computed directly the conductivity at criticality and obtained $t / u$ from the size dependence. Second, using the relation between conductivity and transport properties, we obtained $t / u$ from the diffusion of a random walk on the backbone of the cluster. From both methods, we estimated $t / u$ to be $0.986 pm 0.012$, $0.877 pm 0.014$, $0.785 pm 0.015$, and $0.658 pm 0.030$, for q=1, 2, 3, and 4, respectively. We also evaluated $t / u$ for non integer values of $q$ and propose the following conjecture $40gt/ u=72+20g-3g^2$ for the dependence of the reduced conductivity exponent on $q$, in the range $ 0 leq q leq 4$, where $g$ is the Coulomb gas coupling.
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