Abstract
We report transport measurements using pulsed magnetic fields to suppress the superconducting fluctuations (SCF) conductivity in a series of YBa2Cu3O6+x samples. These experiments allow us altogether to measure the temperature Tc' at which SCF disappear, and the pseudogap temperature T*. While the latter are consistent with previous determinations of T*, we find that Tc' is slightly larger than similar data taken by Nernst measurements. A careful investigation near optimal doping shows that T* becomes smaller than Tc', which is an unambiguous evidence that the pseudogap cannot be assigned to preformed pairs. Studies of the incidence of disorder on both Tc' and T* allow us to propose a phase diagram including disorder which explains most observations done in other cuprate families, and to discuss the available knowledge on the pseudogap line in the phase diagram.