The resistivity in the ferromagnetic state of flux-grown
La_{2/3}(Pb,Ca)_{1/3}MnO_3 single crystals, measured in magnetic fields up to 7
T, reveals a strong quadratic temperature dependence at and above 50 K. At
lower temperatures, this contribution drops precipitously leaving the
resistivity essentially temperature independent below 20 K. The Seebeck
coefficient also reflects a change of regime at the same temperature. We
attribute this behavior to a cut-off of single magnon scattering processes at
long wavelengths due to the polarized bands of a double-exchange ferromagnet.Comment: 10 pages, TeX, 4 figures. Revised version. Submitte