  We use magnetic field and ion moment data from the MFI and SWE instruments
onboard the Wind spacecraft to study the nature of solar wind turbulence at
ion-kinetic scales. We analyze the spectral properties of magnetic field
fluctuations between 0.1 and 5.5 Hz over 2012 using an automated routine,
computing high-resolution 92 s power and magnetic helicity spectra. To ensure
the spectral features are physical, we make the first in-flight measurement of
the MFI `noise-floor' using tail-lobe crossings of the Earth's magnetosphere
during early 2004. We utilize Taylor's hypothesis to Doppler-shift into the
spacecraft frequency frame, finding that the spectral break observed at these
frequencies is best associated with the proton-cyclotron resonance scale,
$1/k_c$, compared to the proton inertial length $d_i$ and proton gyroscale
$\rho_i$. This agreement is strongest when we consider periods where
$\beta_{i,\perp}\sim1$, and is consistent with a spectral break at $d_i$ for
$\beta_{i,\perp}\ll1$ and $\rho_i$ for $\beta_{i,\perp}\gg1$. We also find that
the coherent magnetic helicity signature observed at these frequencies is
bounded at low frequencies by $1/k_c$ and its absolute value reaches a maximum
at $\rho_i$. These results hold in both slow and fast wind streams, but with a
better correlation in the more Alfv\'enic fast wind where the helicity
signature is strongest. We conclude that these findings are consistent with
proton-cyclotron resonance as an important mechanism for dissipation of
turbulent energy in the solar wind, occurring at least half the time in our
selected interval. However, we do not rule out additional mechanisms.
