  We systematically studied in-plane optical conductivity of
FeSe$_{1-x}$Te$_{x}$ thin films fabricated on CaF$_{2}$ substrates for $x$ = 0,
0.1, 0.2, and 0.4. This system shows a large enhancement of superconducting
transition temperature $T_{\mathrm{c}}$ at $x \sim$ 0.2 and a gentle decrease
in $T_{\mathrm{c}}$ with further increasing $x$. The low-energy optical
conductivity spectrum is described by the sum of narrow and broad Drude
components, associated with coherent and incoherent charge dynamics,
respectively. With increasing Te content, the spectral weight of the narrow
Drude component decreases, while the total weight of the two Drude components
increases. As a consequence, the fraction of the narrow Drude weight
significantly decreases, indicating that Te substitution leads to stronger
electronic correlations. Below the nematic transition temperature, the narrow
Drude weight decreases with decreasing temperature. This indicates the
reduction of the coherent carrier density, resulting from the Fermi-surface
modification induced by the development of the orbital order. The reduction of
the narrow Drude weight with temperature stopped at $x \sim$ 0.2, corresponding
to the disappearance of the nematic transition. Our result suggests that the
increase in the coherent carrier density induced by the suppression of the
nematic transition gives rise to the enhancement of $T_{\mathrm{c}}$. The
decrease in $T_{\mathrm{c}}$ with further Te substitution likely arises from
too strong electronic correlations, which are not favorable for
superconductivity.
