  We investigate in-plane quasiparticle tunneling across thin film grain
boundary junctions (GBJs) of the electron-doped cuprate
La$_{2-x}$Ce$_{x}$CuO$_4$ in magnetic fields up to $B=16 $T, perpendicular to
the CuO$_2$ layers. The differential conductance in the superconducting state
shows a zero bias conductance peak (ZBCP) due to zero energy surface Andreev
bound states. With increasing temperature $T$, the ZBCP vanishes at the
critical temperature $T_c\approx29 $K if B=0, and at $T=12 $K for B=16 T. As
the ZBCP is related to the macroscopic phase coherence of the superconducting
state, we argue that the disappearance of the ZBCP at a field $B_{ZBCP}(T)$
must occur below the upper critical field $B_{c2}(T)$ of the superconductor. We
find $B_{ZBCP}(0) \approx 25 $T which is at least a factor of 2.5 higher than
previous estimates of $B_{c2}(0)$.
