The electrical properties of Barium Titanate 60 nm thick layers grown by Molecular Beam Epitaxy has been probed at the nanoscale by Conductive-Atomic Force Microscopy in Ultra-High Vacuum and in air, and the results compared to electrical measurements obtained on large electrodes. The role of the atmosphere of measurement is clearly highlighted by the absence of resistive memory effect when the measurements are operated in air. Considering the possibly high concentration of oxygen vacancies in such samples, we conclude that the difference might be explained by the annihilation of the oxygen vacancies by the oxygen of the environment. This emphasises the need to control precisely the atmosphere of measurement when probing the electrical properties of such thin dielectric films by atomic force microscope.