  The superior intrinsic properties of graphene have been a key research focus
for the past few years. However, external components, such as metallic
contacts, serve not only as essential probing elements, but also give rise to
an effective electron cavity, which can form the basis for new quantum devices.
In previous studies, quantum interference effects were demonstrated in graphene
heterojunctions formed by a top gate. Here phase coherent transport behavior is
demonstrated in a simple two terminal graphene structure with clearly-resolved
Fabry-Perot oscillations in sub-100 nm devices. By aggressively scaling the
channel length down to 50 nm, we study the evolution of the graphene transistor
from the channel-dominated diffusive regime to the contact-dominated ballistic
regime. Key issues such as the current asymmetry, the question of Fermi level
pinning by the contacts, the graphene screening determining the heterojunction
barrier width, the scaling of minimum conductivity and of the on/off current
ratio, are investigated.
