  Nearly a 50% decrease of resistivity \rho (T,x) due to just 4% Cu doping on
the Mn site of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) is observed. Attributing the
observed phenomenon to the substitution induced decrease of the spin polaron
energy E_s(x) below the Curie point T_C(x)=T_C(0)(1-x), all data are found to
be well fitted by the nonthermal coherent tunneling expression \rho (T,x) =
\rho_0*exp(-\gamma M^2(T,x)) assuming
M(T,x)=M_R(x)+M_0(x)*tanh{\sqrt{[T_C(x)/T]^2-1}} for the magnetization in the
ferromagnetic state. The best fits through all the data points yield M_0(x)=
\sqrt(1-x)M_0(0), M_R(x)=\sqrt(x)M_0(0), and E_s(x)=E_s(0)(1-x)^4 for the Cu
induced modifications of the Mn spins dominated zero-temperature spontaneous
magnetization, the residual paramagnetic contribution, and spin-polaron energy,
respectively, with E_s(0)=0.12 eV.
