  We have investigated the in-plane thermal conductivity $\kappa_{ab}(T,H)$ of
large single crystals of optimally oxygen-doped
(Y$_{1-x}$,Sm$_{x}$)Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ ($x$=0, 0.1, 0.2 and 1.0)
and YBa$_{2}$(Cu$_{1-y}$Zn$_{y}$)$_{3}$O$_{7-\delta}$($y$=0.0071) as functions
of temperature and magnetic field (along the c axis). For comparison, the
temperature dependence of $\kappa_{ab}$ for as-grown crystals with the
corresponding compositions are presented.
  The nonlinear field dependence of $\kappa_{ab}$ for all crystals was observed
at relatively low fields near a half of $T_{c}$. We make fits of the
$\kappa(H)$ data to an electron contribution model, providing both the mean
free path of quasiparticles $\ell_{0}$ and the electronic thermal conductivity
$\kappa_{e}$, in the absence of field. The local lattice distortion due to the
Sm substitution for Y suppresses both the phonon and electron contributions. On
the other hand, the light Zn doping into the CuO $_{2}$ planes affects solely
the electron component below $T_{c}$, resulting in a substantial decrease in
$\ell_{0}$ .
