  Ultrafast electron dynamics in solids under strong optical fields has
recently found particular attention. In dielectrics and semiconductors, various
light-field-driven effects have been explored, such as high-harmonic
generation, sub-optical-cycle interband population transfer and nonperturbative
increase of transient polarizability. In contrast, much less is known about
field-driven electron dynamics in metals because charge carriers screen an
external electric field in ordinary metals. Here we show that atomically thin
monolayer Graphene offers unique opportunities to study light-field-driven
processes in a metal. With a comparably modest field strength of up to 0.3
V/{\AA}, we drive combined interband and intraband electron dynamics, leading
to a light-field-waveform controlled residual conduction current after the
laser pulse is gone. We identify the underlying pivotal physical mechanism as
electron quantum-path interference taking place on the 1-femtosecond
($10^{-15}$ second) timescale. The process can be categorized as
Landau-Zener-St\"uckelberg interferometry. These fully coherent electron
dynamics in graphene take place on a hitherto unexplored timescale faster than
electron-electron scattering (tens of femtoseconds) and electron-phonon
scattering (hundreds of femtoseconds). These results broaden the scope of
light-field control of electrons in solids to an entirely new and eminently
important material class -- metals -- promising wide ramifications for band
structure tomography and light-field-driven electronics.
