  High-quality quantum-state and entanglement transfer can be achieved in an
unmodulated spin bus operating in the ballistic regime, which occurs when the
endpoint qubits A and B are coupled to the chain by an exchange interaction
$j_0$ comparable with the intrachain exchange. Indeed, the transition amplitude
characterizing the transfer quality exhibits a maximum for a finite optimal
value $j_0^{opt}(N)$, where $N$ is the channel length. We show that
$j_0^{opt}(N)$ scales as $N^{-1/6}$ for large $N$ and that it ensures a
high-quality entanglement transfer even in the limit of arbitrarily long
channels, almost independently of the channel initialization. For instance, the
average quantum-state transmission fidelity exceeds 90% for any chain length.
We emphasize that, taking the reverse point of view, should $j_0$ be
experimentally constrained, high-quality transfer can still be obtained by
adjusting the channel length to its optimal value.
