Excel process remains running after COM has finished

 using System.Diagnostics;
using System.Runtime.InteropServices;

Excel.Application	excelApp = null;
Excel.Workbook	excelWorkbook = null;
Excel.Sheets	excelSheets = null;
Excel.Worksheet	excelWorksheet = null;
Excel.Workbooks	excelWorkbooks = null;

private void button3_Click(object sender, System.EventArgs e)
{
  excelApp                 = new Excel.ApplicationClass();
  excelApp.Visible  = true;
  excelWorkbooks   = excelApp.Workbooks;
  excelWorkbook = excelWorkbooks.Add(System.Reflection.Missing.Value);
  excelSheets = excelWorkbook.Worksheets;
  excelWorksheet = (Excel.Worksheet)   excelSheets.get_Item(1);
  excelWorksheet.Cells[1,1] = "42";
}

private void button4_Click(object sender, System.EventArgs e)
{
  excelWorkbook.Close (false, System.Reflection.Missing.Value,System.Reflection.Missing.Value) ; 

  excelWorkbooks.Close();
  excelApp.Quit();

  Marshal.ReleaseComObject(excelWorksheet);
  Marshal.ReleaseComObject(excelSheets);
  Marshal.ReleaseComObject(excelWorkbooks);
  Marshal.ReleaseComObject(excelWorkbook);
  Marshal.ReleaseComObject(excelApp);

  excelWorksheet = null;
  excelSheets = null;
  excelWorkbooks = null;
  excelWorkbook = null;
  excelApp = null;

  GC.GetTotalMemory(false);
  GC.Collect();
  GC.WaitForPendingFinalizers();
  GC.Collect();
  GC.GetTotalMemory(true);

  Process[] pProcess;
  pProcess = System.Diagnostics.Process.GetProcessesByName("Excel");
  if (pProcess.Length > 0)
  {
     pProcess[0].Kill();
  }

}