  The realization of p-n junctions in graphene, combined with the gapless and
chiral nature of its massless Dirac fermions has led to the observation of many
intriguing phenomena such as quantum Hall effect in bipolar regime, Klein
tunneling, and Fabry-P\'{e}rot interferences all of which involve electronic
transport across p-n junctions. Ballistic snake states propagating along the
p-n junctions have been predicted to induce conductance oscillations,
manifesting their twisting nature. However, transport studies along p-n
junctions have so far only been performed in low mobility devices. Here, we
report the observation of conductance oscillations due to ballistic snake
states along a p-n interface in high quality graphene encapsulated by hexagonal
boron nitride. These snake states are exceptionally robust as they can
propagate over $12$~$\mu$m, limited only by the size of our sample, and survive
up to at least $120$~K. The ability to guide carriers over a long distance
provide a crucial building block for graphene-based electron optics.
