  The negative Hall constant R_H measured all over the phase diagram of
Ba(Fe(1-x)Cox)2As2 allows us to show that electron carriers always dominate the
transport properties. The evolution of R_H with x at low doping (x<2%)
indicates that important band structure changes happen for x<2% prior to the
emergence of superconductivity. For higher x, an change of the electron
concentration with T is required to explain the low T variations of R_H, while
the electron scattering rate displays the T^2 law expected for a Fermi liquid.
The T=0 residual scattering is affected by Co disorder in the magnetic phase,
but is rather dominated by incipient disorder in the paramagnetic state.
