  Transport properties of high transition temperature (high Tc) cuprate
superconductors are investigated within a two-band model. The doping dependent
Hall coefficients of La_{2-x}Sr_xCuO_4 (LSCO) and Nd_{2-x}Ce_xCuO_4 (NCCO) are
explained by assuming the coexistence of two carriers with opposite charges,
loosely speaking electrons (e) and holes (h). Such a possible electron-hole
coexistence (EHC) in other p-type cuprates is also inferred from subtle
features in the Hall coefficient R_H and thermopower S. The EHC possibly
relates to the pseudogap and sign reversals of transport coefficients near Tc.
It also corroborates the electronlike Fermi surface revealed in recent
photoemission results. An experimental verification is proposed.
