  We fabricate and investigate high quality graphene devices with contactless,
suspended top gates, and demonstrate formation of graphene pnp junctions with
tunable polarity and doping levels. The device resistance displays distinct
oscillations in the npn regime, arising from the Fabry-Perot interference of
holes between the two pn interfaces. At high magnetic fields, we observe
well-defined quantum Hall plateaus, which can be satisfactorily fit to
theoretical calculations based on the aspect ratio of the device.
