  We investigate the dynamics of correlated charge carriers in the vicinity of
the Mott metal-to-insulator transition in various of the title
quasi-two-dimensional organic charge-transfer salts by means of fluctuation
(noise) spectroscopy. The observed $1/f$-type fluctuations are quantitatively
very well described by a phenomenological model based on the concept of
non-exponetial kinetics. The main result is a correlation-induced enhancement
of the fluctuations accompanied by a substantial shift of spectral weight to
low frequencies close to the Mott critical endpoint. This sudden slowing down
of the electron dynamics may be a universal feature of metal-to-insulator
transitons. For the less correlated metallic materials, we find a
crossover/transition from hopping transport of more-or-less localized carriers
at elevated temperatures to a low-temperature regime, where a metallic coupling
of the layers allows for coherent interlayer transport of delocalized
electrons.
