  A model-independent partial-wave analysis of the \swave component of the
$K\pi$ system from decays of $D^{+}$ mesons to the three-body $\Km\pip\pip$
final state is described. Data come from the Fermilab E791 experiment.
Amplitude measurements are made independently for ranges of $\Km\pip$ invariant
mass, and results are obtained below 825 \MeVcc, where previous measurements
exist only in two mass bins.
  This method of parametrizing a three-body decay amplitude represents a new
approach to analysing such decays. Though no model is required for the \swave,
a parametrization of the relatively well-known reference \pdash and \dwave s,
optimized to describe the data used, is required. In this paper, a Breit-Wigner
model is adopted to describe the resonances in these waves.
  The observed phase variation for the \sdash, \pdash and \dwave s do not match
existing measurements of $I=\half$ $\Km\pip$ scattering in the invariant mass
range in which scattering is predominantly elastic.
  If the data are mostly $I=\half$, this observation indicates that the Watson
theorem, which requires these phases to have the same dependence on invariant
mass, may not apply to these decays without allowing for some interaction with
the other pion.
  The production rate of $\Km\pip$ from these decays, if assumed to be
predominantly $I=\half$, is also found to have a significant dependence on
invariant mass in the region above 1.25 \GeVcc.
  These measurements can provide a relatively model-free basis for future
attempts to determine which strange scalar amplitudes contribute to the decays.
