read.neuron.swc reads an SWC file on disk into a fully
  parsed neuron representation. However we normally recommend
  using read.neuron(s) since those functions cope
  with any file type.
read.ngraph.swc reads an SWC file on disk into the more 
  generic (and forgiving) ngraph representation which provides
  a bridge to the igraph library.
read.neuron.swc(f, ...)
read.ngraph.swc(f, weights = FALSE, directed = TRUE, ...)path to file
Additional arguments. read.neuron.swc passes these to
as.neuron and then on to neuron.
read.neuron.swc passes them to ngraph.
Logical value indicating whether edge weights defined by the
3D distance between points should be added to graph (default FALSE)
or a numeric vector of weights.
Whether the resultant graph should be directed (default TRUE)
These functions will accept SWC neurons with multiple trees and
  arbitrary point index order. However only read.ngraph.swc will
  accept SWC files with cycles.
These functions would normally be called from
  read.neuron(s) rather than used directly. The
  only benefit of using read.neuron.swc is to avoid a very small
  overhead in identifying the SWC file type. Note that only
  read.neurons can read many files in one command to construct
  a neuronlist object.
According to http://www.neuronland.org/NLMorphologyConverter/MorphologyFormats/SWC/Spec.html SWC file format has a radius not a diameter specification