  This work, together with \cite{KrumWica} and \cite{KrumWicc}, forms a series
of articles devoted to an analysis of interior singularities of locally area
minimizing $n$-dimensional rectifiable currents $T$ of codimension $\geq 2$. In
the present article we establish a new height estimate for $T$, which says that
in a cylinder in the ambient space, the pointwise distance of $T$ to a union of
non-intersecting planes is bounded from above, in the interior, \emph{linearly}
by the $L^{2}$ height excess of $T$ relative to the same union of planes,
whenever appropriate smallness-of-excess conditions are satisfied. We use this
estimate and techniques inspired by the works \cite{Sim93}, \cite{Wic14},
\cite{KrumWic2} to establish a decay estimate for $T$ whenever, among other
requirements, $T$ is significantly closer to a union of planes meeting along an
$(n-2)$-dimensional subspace than to any single plane. Combined with
\cite[Theorem~1.1]{KrumWica}, this implies two main results: (a) $T$ has a
unique tangent cone at ${\mathcal H}^{n-2}$ a.e.\ point, and (b) the set of
singular points of $T$ where $T$, upon scaling, does not decay \emph{rapidly}
to a plane is countably $(n-2)$-rectifiable. In particular, concerning
\emph{branch points} of $T$, the work here and in \cite{KrumWica} establishes
the fact that rapid decay to a unique tangent plane is the generic behaviour,
in the sense that at ${\mathcal H}^{n-2}$ a.e.\ branch point, $T$ decays to a
unique tangent plane and has \emph{planar frequency} (or the order of contact
with the tangent plane) bounded below by $1 + \alpha$ for some fixed $\alpha
\in (0, 1)$ depending only on $n$, $m$ and a mass upper bound for $T$; the
planar frequency exists, is uniquely defined and is finite by the approximate
monotonicity of the (intrinsic) planar frequency function introduced in
\cite{KrumWica}.
