We provide a direct proof that the Haagerup estimate on the
completely bounded norm of elementary operators is best
possible in the case of $\mathcal{B}(H)$ via a generalisation
of a theorem of Stampfli. We show that for an elementary
operator $T$ of length $\ell$, the completely bounded norm is
equal to the $k$-norm for $k = \ell$. A $C$*-algebra $A$ has
the property that the completely bounded norm of every
elementary operator is the $k$-norm, if and only if $A$ is
either $k$-subhomogeneous or a $k$-subhomogeneous extension of
an antiliminal $C$*-algebra.