  We present in-plane optical study on Tl$_{2}$Ba$_{2}$CuO$_{6+\delta}$ single
crystals with substantially different $T_c$. The study reveals that the
overdoping does not lead to a further increase of the carrier density but a
decrease of scattering rate. The most significant change occurs at low
temperature and in the low frequency. A characteristic spectral feature, seen
most clearly for the optimally doped sample and commonly ascribed to the mode
coupling effect, weakens with doping and disappears in the heavily overdoped
sample. Meanwhile, the optical scattering rate evolves from a linear-$\omega$
dependence to an upward curvature lineshape. Both the temperature and frequency
dependence of the scattering rate can be described by a power law relation. We
elaborate that the overall decrease of the optical scattering rate originates
from the increase of both the quasiparticle life time and the Fermi velocity
near the ($\pi$,0) region in the Fermi surface.
