  The transport and optical properties of the Nb-doped
Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$ with x = 0.03 and 0.07 have been
investigated and compared with those of the undoped CsV$_{3}$Sb$_{5}$. Upon Nb
doping, the charge-density wave (CDW) transition temperature $T_{\text{CDW}}$
is suppressed, and the superconducting temperature $T_{c}$ rises. The residual
resistivity ratio decreases with Nb doping, suggesting an increase of disorder.
For all compounds, the optical conductivity in the pristine phase reveals two
Drude components (D1 and D2). The substitution of Nb causes an increase of D1
alongside a reduction of D2 in weight, which implies a change of the Fermi
surface. The total Drude weight is reduced with increasing Nb content,
signifying an enhancement of electronic correlations. Below $T_{\text{CDW}}$,
while the optical conductivity clearly manifests the CDW gap in all materials,
the gapped portion of the Fermi surface shrinks as the Nb content grows. A
comprehensive analysis indicates that the change of the Fermi surface, the
enhancement of electronic correlations, the shrinkage of the removed Fermi
surface by the CDW gap, and the increase of disorder may all have a
considerable impact on the interplay between the CDW and superconductivity in
Cs(V$_{1-x}$Nb$_{x}$)$_{3}$Sb$_{5}$.
