Thermoelectric power, electrical resistivity and magnetization experiments,
performed in the paramagnetic phase of La_{2/3}Ca_{1/3}MnO_3, provide evidence
for polaron-dominated conduction in CMR materials. At high temperatures, a
large, nearly field-independent difference between the activation energies for
resistivity (rho) and thermopower (S), a characteristic of Holstein Polarons,
is observed, and ln(rho) ceases to scale with the magnetization. On approaching
T_c, both energies become field-dependent, indicating that the polarons are
magnetically polarized. Below T_c, the thermopower follows a law S(H) prop.
1/rho (H) as in non saturated ferromagnetic metals.Comment: 10 pages, 5 .gif figures. Phys. Rev B (in press