  Resistivity measurements have been performed on a low (LR)- and high
(HR)-resistance variant of the kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br superconductor.
While the HR sample was synthesized following the standard procedure, the LR
crystal is a result of a somewhat modified synthesis route. According to their
residual resistivities and residual resistivity ratios, the LR crystal is of
distinctly superior quality. He-gas pressure was used to study the effect of
hydrostatic pressure on the different transport regimes for both variants. The
main results of these comparative investigations are (i) a significant part of
the inelastic-scattering contribution, which causes the anomalous rho(T)
maximum in standard HR crystals around 90 K, is sample dependent, i.e.
extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a
strongly temperature-dependent behavior at T > T* to an only weakly T-dependent
rho(T) at T < T* is unaffected by this scattering contribution and thus marks
an independent property, most likely a second-order phase transition, (iii)
both variants reveal a rho(T) proportional to AT^2 dependence at low
temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent
coefficients A and upper bounds for the T^2 behavior measured by T_0. The
latter result is inconsistent with the T^2 dependence originating from coherent
Fermi-liquid excitations.
