ivysettings.xmlfile to configure Ivy (what else?).
ivy.xmland doesn't do much beyond that if you are only using Ivy to upload your artifacts.
ivy:publishpublishes the artifcats to Nexus.
An example build file combining those steps that expects you to provide the authentication information via the command line (i.e.
ant -Dupload.user=YOUR-ASF_ID -Dupload.password=YOUR-PASSWORD) can be found here: http://svn.apache.org/repos/asf/ant/antlibs/common/trunk/upload.xml