We report on the evolution of the coherent electronic transport through a gate-defined constriction in a high-mobility graphene device from ballistic transport to quantum Hall regime upon increasing the magnetic field. At low field, the conductance exhibits Fabry-Perot resonances resulting from the npn cavities formed beneath the top-gated regions. Above a critical field $B^*$ corresponding to the cyclotron radius equal to the npn cavity length, Fabry-Perot resonances vanish and snake trajectories are guided through the constriction with a characteristic set of conductance oscillations. Increasing further the magnetic field allows us to probe the Landau level spectrum in the constriction, with distortions due to the combination of confinement and de-confinement of Landau levels in a saddle potential. These observations are confirmed by numerical calculations.
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